This invention relates generally to filters used for UHF (ultrahigh frequency) and/or SHF (superhigh frequency) band, and more particularly, the present invention relates to coaxial filters which comprise a plurality of coaxial resonators.
Since coaxial resonators, which are small in size and have a high value of unloaded Q, can be readily designed, they have been used as the resonators of oscillators, filters or the like in UHF and SHF bands for a long time. Meanwhile, the performance of semiconductor elements has been remarkably improved, and accordingly, various active elements have been made in the form of solid-state elements. As various elements are made of solid-state elements, demands for providing smaller devices have been increased. As a result, it is now required to provide smaller oscillators, filters or the like which are suitable for such small devices. For this reason, it is a technical problem in these days to provide a small resonator which constitute such circuits.
When reducing the size of a resonator, attention has to be given so that unloaded Q value is not deteriorated because coaxial resonators is apt to suffer from deterioration in unloaded Q value, increasing loss when the size thereof is simply reduced.
The inventors of the present invention devised a coaxial resonator having new structure so that the size thereof is small and the value of Q is high, and proposed this new coaxial resonator in Japanse Patent Provisional publication No. 55-100701. This resonator comprises a bottomed metallic cylinder attached to one end of the center conductor received in a bore of an outer conductor. Although this new coaxial resonator is more superior than the conventional coaxial resonators when the resonator is used alone, the inventors have realized that the coaxial resonator is not suitable for constituting a filter because the value of Q lowers when a plurality of such coaxial resonators are combined and built in a casing. Therefore, when a filter is constructed of a plurality of such resonators, the value of Q of the entire filter is very low.